Utilities/Sphinx: Restore compatibility with Sphinx pre-1.2

Since commit v3.8.0-rc2~28^2~2 (Utilities/Sphinx: Port cmake extension
to Sphinx 1.4, 2017-02-09) we use the `sphinx.version_info` tuple.
However, it was added in Sphinx v1.2 so the check breaks compatibility
with older versions. Revise our check to assume Sphinx pre-1.2 if the
version tuple does not exist.